The erabutoxins are low molecular weight single chain proteins from the sea snake Laticauda semifasciata: they are members of a family of snake venom neurotoxins which like mode. The structures of erabutoxin b and, by analogy, of the related erabutoxins a and c have been established by X-ray crystal structure analysis. Refinement of the structure to 1.5 A resolution is now begun and will continue. Phase refinement procedures of Sayre and of Agarwalare being modified for the space group of this protein and for the Columbia University Computer capacity. Other refinement techniques will also be employed. Postsynaptic neurotoxins of land snakes include a class of long chain toxins with a fifth disulfide bridge. The erabutoxin structure as established suggests that a reactive site may be recognizable. The assumption of topological molecular stability thus implied (that is confidence in the functional importance of the three dimensional structure we have found) is attractive. But structure studies must be undertaken of other short chain and one or more longer chain toxins in order to establish whether a common in vitro structure exists. These studies we propose to undertake. The final stage in an understanding of the detailed interaction between these neurotoxins and the acetylcholine receptor depends upon crystal structure analysis of the receptor inhibitor complex. We propose to attempt crystallization and X-ray crystal studies of the toxin-receptor complex when receptor or complex becomes available.